Introduction: This is an extensive revision of our prior application which makes better use of the opportunities of the Danish longitudinal registers and gives us a robust power for testing our highly novel hypothesis that aeroallergens and allergy are associated with suicide. Issues of temporal and topographic clustering have been addressed, the focus of the design has been tightened and more details are given regarding the analysis. Background: Seasonal peaks of suicide in spring have been consistently reported but the origin is poorly understood. Benefiting from a natural experiment (the abrupt rise of tree-pollen in spring), and considering the high prevalence of allergic sensitization (40-50%), we proposed that the massive release of tree-pollen in spring, via mediators of inflammation, elevates the relative risk for suicide, by affecting individuals with vulnerability for allergies, depression, and suicide. The aims of the study are 1) to confirm the relationship between allergen peaks and suicide peaks (using an ecologic approach and a generalized linear model allowing us to address geographic and temporal clustering), 2) to estimate a predictive association between allergy and suicide, with adjustment for and interaction with variables previously described to be related to suicide, such as diagnosis of mood disorders, prior suicide attempts, and other demographic /socioeconomic variables related to suicide risk, using a nested case control design and conditional logistic regression models , and 3) to estimate the predictive association between spring allergy, and spring suicide during peak tree-pollen periods in interaction with history of mood disorders using conditional logistic regression models. Secondary aims are to analyze specifically the predictive association between allergy and mood disorders and suicidal attempts in individuals and families, to evaluate interactions with demographic and socioeconomic factors, and to estimate protective versus risk elevating effects of (anti-allergy and psychotropic) medications. Implications: The study is poised to identify a potential novel vulnerability for suicide, i.e., allergic sensitization, and a potential novel trigger, aeroallergen exposure, and, in the process, to replicate the previously reported gender-specific relationship between allergy and mood disorders, in individuals and families. Confirming that allergens and allergy are related to suicide epidemiologically, in conjunction with clinical research and laboratory work, is expected to improve prediction and prevention of suicide.